triiodothyronine (T3, liothyronine, Cytomel)

Pathology

• diminished peripheral conversion of T4 -> T3 can result in euthyroid sick syndrome

Biochemistry

• T3 is formed (via activity of thyroid peroxidase) & stored as an iodinated residue of thyroglobulin
• normal thyroid gland makes 8 ug of T3/day
• T3 is preferentially synthesized by the thyroid when availability of iodide is low
• monodeiodination of T4, mainly by the liver, normally produces 32 ug of T3/day.
• 99.7% of T3 in protein bound
• T3 is more active than T4, estimates range form 3-100 fold more active
• treatments that decrease peripheral conversion of T4 -> T3:
  • propranolol
  • propylthiouracil (methimazole does not)
  • glucocorticoids (dexamethasone 4-8 mg/day)

Pharmacology

Tradename: Cytomel.

Indications

• hypothyroidism, thyroid replacement
  • cretinism, myxedema, myxedema coma
• management of non-toxic goiter
• chronic lymphocytic thyroiditis
• adjunct agent in thyroiditis
• thyroid carcinoma
• diagnostic aid Start 25 ug PO QD, max 100 ug/day.

Tabs: 5, 25, 50 ug.

Adverse effects

• uncommon (< 1%)
  • palpitations, nervousness, sweating, tachycardia, headache, cardiac arrhythmias, tremors, insomnia, fever, hair loss, weight loss, increased appetite, diarrhea, abdominal cramps, changes in menstrual cycle, constipation, muscle aches, chest pain, shortness of breath, clumsiness

• Toxicity:
  • serum TSH low
    • serum free T4 is not elevated

Drug interactions

warfarin

• drug interaction(s) of beta-adrenergic receptor antagonists with thyroid hormone

Mechanism of action

• synthetic hormone identical to T3
• thyroid hormones increase metabolic rate of tissues
• involved in regulation of growth & differentiation

More general terms

• thyronine
• thyroid hormone
• thyroid agent

Additional terms

• reverse T3; reverse triiodothyronine
• T3 total in serum/plasma
• thyroglobulin; Tg (TG)
• thyroid peroxidase
• triiodothyronine [T3] (free) in serum
• triiodothyronine [T3] receptor or erbA proto-oncogene protein

Component of

• levothyroxine/triiodothyronine
• thyroxine/triiodothyronine (Euthroid, Thyrolar)

Database

• PubChem: http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=861
• PubChem: http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=5920
• PubChem: http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=5919
• PubChem: http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=71212